Transmission circuits



Fbc.i 23 R926.. Lwl

W. L. CASPER I TRANSMISSION CIRCUITS Filed May 25, 1922 Panarea vFa. 23, 192e.

' rUNITI-:D'ISTATas PATENT OFFICE.

f L. CASPER, or BROOKLYN, NEW Yonx, AssiGNon rro WESTERN `aimerait: f COMPANY, INOonrOnATED, or Naw Yonx, N. Y., A CORPORATION or NEW YORK.

TRANSMISSION CIRCUITS.

Application med may as,

l To wluna 'it may concern.'

` Be it known that I, WILLIAM L. CASPER, a citizen of the UnitedStates of America residing at Brooklyn, in the county of Kings and State of New York, have invented. certain new and useful Improvements in Transmission Circuits, of. which the following is a full, clear, concise, and exact description.

This invention relates tov transmission circuits such as loud speaking amplifier circuits.

One objectof this invention is to provide an amplifier-and a loud.. speaking receiver operating without substantial distortion.

Another object is to-fprovide means for overcoming -the variable'. sen'sitiveness with frequency of a translating device such as a telephone receiver. r

Another object is to provide a trans- 'former between an incoming line and a receiver such that the distortion of the repeated signals due to the receiver is substantially neutralized by distortion in the transformer.

In accordance with the embodiment of this invention hereinafter described in detail, a telephone receiver is coupled to an incoming line by means of a repeating coil or ,transformer so designed that the transformer distorts the incoming signals in an opposite'sense from the distortion due to the receiver whereby the sounds emitted by the receiver are substantially a faithful representation of the si nals received from the line. An electric dlscharge amplifier of the three-electrode type may be employed between the line and the receiver if desired to increase the 4volume of the sounds emitted.

\ It has been found that the combined amplifying horn and receiver, such as are used in`loud speaking systems, have a falling sensitive-ness for frequencies in -the upper 'range of y frequencies of importance in speech. That is,-the amplitude of the vibration of the diaphragm instead of remaining substantia ly constant over the range of frequencies of importance in speech, tends to decrease at the higher frequencies giving a drooping characteristic.

In accordance with this invention, it has been found that this drooping effect can be substantially neutralized by employing with 1922. Serial No. 563,081.

the receiver a transformer designed so that for the upper speech frequencyrange thetransmission through the. transformer has a rising characteristic compared with the lower and middle speech frequency range. The signals to be translated into sounds therefore have the frequencies of the upper speech range overemphasized bythe transformer and underemphasized by the loud speakin receiver whereby the distorting effect o each is substantially neutralized.

In my Patents Nos. 1,530,648 and 1,530,649 dated March 24,1925, have' been described various means for giving a transformer a flat transmission characteristic or a rising transmission characteristic. As described therein, the transmission characteristic of a transformer is dependent upon the intensity of the capacity effects between the transformer wlndings and between the coils of each' winding. An linductance device may be connected in series with the transformer windings of such a value as to resonate with the capacity effects to give a transmission characteristic which may vary with Vthe frequency in an desired manner. If the inductance device chosen is of such a value that the resonance peak of the transmission curve is aboveI the frequency range of importance in speech, it follows that the transmission curve of the transformer has a. rising slope for the upper speech range. The resonance peak should be adjusted, of course, until the rising characteristic of the transformer substantially offsets or neutralizes the falling characteristic of the amociated horn and receiver. Instead of Obtaining the resonance effect as above described, it has been found, as also pointed out in the above applications, that the effect of the series inductance element may be simulated by adjusting the leakage reactance of the transformer to an appreciable amount, since leakage reactance in a transformer for this purpose is the equivalent of an inductance in series with the primary and `secondary windings.

In the event that a vacuum tube amplifier is employed between the transformerv and the receiver, the capacity effects between the electrodes of the amplifier, epecially between the grid and the cathode, should also be taken into consideration in designing the correcting transformer so that the rislng fier and the transformer.

characteristic curve represents the resultant `transmission ofthe amplifier and the transformer due to the leakage reactance resonating with capacity effects both in the amplirlhese capacity effects between the electrodes of a vacuum tube have been discussed in detail in the above noted applications.

Referring to the drawings, Fig. 1 illustrates this invention embodied in a twostage amplifier working into a loud speaking receiver wherein energy for the amplifiers is obtained from storage batteries; Fig. 9 represents how Fig. 1 may be modified to energize the filaments of the amplifiers from alternating current; Fig. 3 illustrates how both the anode-cathode voltage .and heating current may be obtained from an alternating current source of voltage; and Fig. 4 illustrates the transmission curves of a combined loud speaking receiver and horn and of the combined transformer and amplifier circuit of Figs. 1 or 3.

The loudspeaking amplifier circuit of Fig. 1 comprises a vacuum tube amplifier 10 working into a push-pull amplifier 11, 12 for receiving and amplifying signals from `the incoming line 13, 13 and impressing the amplified signals upon a suitable loud speaking receiver 14 attached to a suitable loud speaking horn l5. v

The input circuit of amplifier 10 is connected to line 13 by a transformer 16, the secondary winding of which is provided with a plurality of taps cooperating with a contact arm 17 whereby the effective gain of the amplifiers may be regulated. Contact arm 17 in the position shown in the drawing gives, ofcourse, the maximum gain of the receiving circuit. The output circuit of amplifier 10 is coupled to the input circuits of tubes l1 and` 12 by a transformer '18, the secondary winding of which has a midpoint connection to the 'combined cathodes of tubes 11 and 1 2, the outside terminals of the secondary winding being con- -nected to the respective grids or control electrodes. rihe combined output circuits of tubes 11 and 12' are coupled to the loud speaking receiver 14 by a suitable transformer 19.. Space current for the tubes 10, 11 and 12 may be obtained by a battery 20 which has its negative terminal connected to the cathodes and its positive terminal connected through primary winding of transformer 18 to the anode of tube 10 and through the two halves of the primary winding of l'transformer 19 to the respective anodes cf tubes i1 and 12. Heating carren'tfor the filaments is supplied from a battery 21, which heats the filaments ef tube trausfc obvious connections to a source of voltage 22.

The type of receiver-.employed in connection with the amplifiers may, for example, be similar to the structure illustrated and claimed in the United States patentto Egerton on telephone apparatus, No. 1,365,898, issued January 1S. 1921.

It has been fo und in general that the'a'mplitude of vibra-tion of the diaphragm of a loud speaking receiver, such as that shown in the Egerton patent, is not constant `with' frequency and the ordinates represent the y amplitude of vibration of the diaphragm. The curve as shown in Fig. 4 'is plotted on a log scale to an arbitrary base. As shown by curve 23, the combined receiver and horn has a falling sensitiveness for frequencies from 1 000 cycles up to the upper range of frequencies of importance in speech, due, in part, to the resonating properties of the diaphragm. as Well as the combined receiver and horn. his drooping characteristic is objectionable since the sounds emitted by the receiver will not be a faithful copy of the incoming si nals since the frequencies in the upper speec 1 range will be underemphasized due to this drooping characteristic. In accordance with this invention, it has been found that this drooping characteristic of the horn and receiver may be substantially neutralized by adjusting the leakage reactancesof transformers 16 and 18 so that the transmission characteristic curve of the amplier and transformer circuit has a rising slope for the upper speech frequency range as shown in curve 24 of Fig. 4, the abscissae for which represent frequency and the ordinates represent miles gain, both plotted on a log scale. When' the transformers employed are of the two-mesh type, with the windings wouutl on the'common limb of the two meshes, a convenient method for making the leakage reactance appreciable is` to reduce the cross section of the common limb to a considerable extent over the cross section it should have in ordinary transformer practice. F or example, appreciable leakage reactance may be obtained by making the cross section of the center common limb, two-thirds of the average cross section of the rest of one of the meshes added to the average cross section of the rest of the second mesh. ilhs has been described and claimed in detaii in the copending United States patent tc F. E. Field, No. 1,507,994 dated September 9, 1924-. The leal-:age reactances of the "t act as inductance eieinents in se priinarj' rind seccndary wind and eguivaient elements resonate with-the internal capacity effects of the -transformers and the internal v capacity eHects between the electrodes of the CTI amplifiers to. give a tuned circuit Whichfmay have 1ts resonance peak at a frequency above the frequency range of importance in speech.

l Ifl 'the leakage yreactances are adjusted to ive the resonance peak above the speech requency range, it follows that the resultant transmission curve for the combined amplifiers and transformers will have a rising `slope from the middle speech frequency range-to the upper speech frequency limit.

lThe various elements of the amplifier circuit may be adjusted to give this slope the proper characteristic and rising characteristic substantially opposite and equal to the falling characteristic-of the combined horn and receiver.. With such an arrangement therefore the combined' transformers and amplifiers will overemphasize the upper speech fre-A quency range an amount substantially equal .l and opposite to the underemphasizing of the upper speech range by the horn and receiver. AThe result of the combination, is therefore,

yto give a .greatly improvedtransmission especially for the upper speech frequency range, and the sounds emitted by the receiver will be a substantially faithful copy of the signals received from'the incoming line 1S.

f In one case the value of this curve for 1000 :cycles represented fifty miles gain, while-the value of the lcurve for 5000 cycles represented fifty-six miles gain.

f As described in they Arnold Patent No.

1,349,252- of August 10, 1920, each of the `amplifiers 11 and 12 should be arranged so that they Work into an output impedance substantially. equal to their internal impedance. If desired,I a ground connection 25 may be provided for the cathodes so that the cathodes may be substantially grounded whereby singing isv prevented.

This invention, ofcourse, is not limited to an amplifierarrangement 1n which the heatl ing current for `the filaments is obtained :from a battery since the energizing voltage foi-the tubes may be obtained by various arrangements. Fig. 2, for example, shows an alternative arrangement in which the cathodes may be connected to terminals 26 and 27 kand heated by alternating current obtained through a transformer 28, the primary winding of which is in circuit with an alternating current source 29. In order toprevent the 'low frequency hum of the generator 29 frombeing impressed upon receiver 14, the anode and grid connections through batteries 20 and 22 should be taken from the midpoint of the secondary winding of transformer 28.

Fig. 3 is an alternative arrangement in .which lthe anode-cathode voltage for the tubes 10, 11 and 12 is obtained by 'rectifying alternating current from a suitable generator 34. As shown in this figure, generator 34 is connected to the amplifiers through a transformer 35. The midpoint of the Secondary Winding of transformer 35 by lead 36 is connected to the anodes of tubes 10, 11 and 12, one'end of the secondary Winding being connected to the cathodes through a two-electrode rectifier 37 and the other end being connected through a two-electrode rectifier 38 to the cathodes. The rectified voltl age obtained through tubes 37 and 38 may be smoothed out by inserting between the amplifier tubes 10, 11 and 12 and the rectifiers a Campbell wave filter 39 of as many sections as desired. Thislfilter should be designed to pass only the direct current component of the rectified voltage. The manner in which the filter should be designed is described in detail in the United States patent to Campbell, No. 1,227,113 of May 22, 1917. Heating current fortheV cathodes of tubes 10, 11 and 12 is obtained in Fig. 3 by connections to an alternating current source 40 which may be similarly connected as in Fig. 2. In addition to connecting the various'gridsand anodes of the amplifier tubes to thc midpoint 41 of the secondary winding of the transformer42, it may be found advisable to also connect thereto by a lead 43, one side of the' secondary winding of transformer 19 which is in circuit with the rel ceiver 14. This connection aids in eliminating the hum in the receiver due to heating the filaments by alternating current.

The above described loud speaking amplifier circuit is of general utility and capable of Wide usel in the art. It is particularly suitable, for example, to be 4employed .in the amplification ofy detected radio frequency signals in which case incoming line 13, 13 may be suitably connected to the output circuit ofva high frequency detector. Various modifications may be made in the above described circuit without'departing in any wise from the spirit of the invention as defined in the appended claims.

The invention claimed is:

1. In combination, a loud speaking rcceiver having a falling characteristic over a given range of speech frequencies, and a 1 arrangement in circuit with said receiver, and` having a rising transmission characteristie over said range.

4. In combination, means having a varying sensitiveiiess for translating electric waves into sound Waves, means comprising a transformer, a plurality of windings for said transformer, said second means having 'a rising transmission characteristic for a given range of frequencies for neutralizing to a substantial extent the varying sensiiveness of said first-mentioned means over said range.

5.1m combination, means having distortion-producing characteristics for translating electric Waves into sound Waves of the speech frequency range, an incoming line, and means including a transformer connect` ing said line with said first means, and having a transmission characteristic resonating at a frequency above the speech range for neutralizing the distortion produced by said first means.

6. ln combination, means having distortion-producing characteristics for translating electric waves into sound waves, and means comprising a transformer of high leakage reactance to give a rising characteristie for overcoming thedistortion produced by said first/'means 7. In combination, means having distortion-producing characteristics for translating electric Waves into energy of .another character, and means comprising a trans-v former of high leakage reactance to give a rising characteristic for overcoming the distortion produced by said first means.

8. In combination, an incoming line, means for translating electric waves from said line into sound waves, said means having a transmission characteristic varying With frequen cy, amplifying means and. atransformer between said line and said first means, said amplifying means and said transformer hav` ing a transmission characteristic varying in the opposite sense to. the variation vof said first means. g

In Witness whereof, I hereunto subscribe my name this' 22nd day of May, A. D. 1922.'

WILLIAM L. CASPER. 

